In a vane pump, a plurality of vanes are housed in radial slits formed in a rotor. Each vane is biased in a projecting direction from the slit by a pressure of a back pressure chamber that presses a base end portion of the vane and a centrifugal force acting thereon as the rotor rotates, and as a result, a tip end portion of the vane slides against an inner peripheral cam surface of a cam ring. As the rotor rotates, the vane sliding against the cam surface performs a reciprocating motion such that a pump chamber expands and contracts, and as a result, working oil is supplied to and discharged from the pump chamber.
JP11-230057A proposes a vane pump in which back pressure grooves communicating with a back pressure chamber and connecting grooves connecting the respective back pressure grooves are formed in front and rear pressure plates provided to sandwich a rotor and respective vanes. In this vane pump, the connecting grooves formed in the front pressure plate are shaped differently to the connecting grooves formed in the rear pressure plate. According to this vane pump, a sealing performance of the pump chamber can be improved.